This invention relates to the lens actuating system of an optical disk drive, and more particularly, to improvements in the lens actuating system that enhances its frequency response characteristics.
An optical disk drive reads or writes information on concentric or spiral tracks on an optical disk by directing a light beam through an objective lens that is movably supported in a movable frame. A focusing servo moves the lens relative to the frame in a direction perpendicular to the surface of the disk, to focus the light beam to a spot on the disk. A seeking servo moves the frame parallel to the surface of the disk in a direction perpendicular to the tracks, to bring the spot to a desired track. A tracking servo moves the lens in the same direction relative to the frame, to keep the spot centered on the desired track as the disk rotates.
These servos conventionally operate by using light reflected from the disk to detect the position of the lens relative to the disk surface, but reliance on that scheme alone has certain inherent disadvantages. With regard to motion parallel to the disk surface, there is mutual interference between the seeking and tracking servos. For example, unwanted motion generated by the tracking servo can delay the seeking of the desired track. Disabling the tracking servo during seeking is not a satisfactory solution, because the tracking servo cannot hold the lens still relative to the frame and the lens tends to drift in response to random external disturbances.
Similarly, since the focusing servo does not know the position of the lens relative to the frame, the lens assembly must be attached to the frame by flexible supports or dampers that hold the lens in a fixed neutral position when the focusing servo is inactive. Such supports or dampers are difficult to position accurately during the manufacturing process, however, and have unwanted vibrational modes that can disrupt focusing and tracking control.
The above problems show up as disturbances in the frequency response characteristics of the servo control systems. Additional disturbances in these frequency characteristics have been found to arise from improper balance of the lens assembly, resonant vibration of the disk excited by movement of the lens, and competition between the tracking and seeking servos for control of the lens when the gain curves of these two servos intersect at more than one point. All these disturbances destabilize servo control and degrade the tracking, seeking, and focusing performance of the optical disk drive.